With the introduction of multi-carrier operation in GSM EDGE Radio Access network, GERAN, the number of RLC blocks that can be transmitted over a given transmission time interval is increased by a factor that is equivalent to the number of carriers. This means that the transmit window size, WS, needs to be increased to accommodate an increased number of RLC blocks being put “on flight” so that a stalling condition due to the WS limitation can be avoided, i.e. a condition where no new RLC Packet Data Units, PDU, can transmitted even if available.
However, this requires also that the Block Sequence Number, BSN, space used to address the RLC blocks is consequently increased, as now more RLC blocks can be transmitted per transmission time interval, TTI, as per the increased WS.
In general, for protocols operating in Acknowledged Mode, AM, there is a need for feedback communication between the receiving to the transmitting entity. This is typically realized by indexing the blocks being transmitted using a block sequence number, BSN, wherein the receiving entity can indicate which blocks were received or not by sending an ACK/NACK, Acknowledgement/Negative Acknowledgement, back to the transmitting entity using BSN related references. To limit the range of block indices, a Sequence Number Space, SNS, size limitation is imposed. For any given SNS, a corresponding Window Size, WS, is associated which effectively establishes a maximum BSN distance between the BSN of the oldest outstanding, not yet acknowledged, block transmitted and the BSN of the next RLC block pending transmission, i.e. that has not yet been transmitted. Once this WS limitation is reached, the transmitting entity experiences stalling and will therefore not transmit any new blocks until it receives reception confirmation of the oldest outstanding block and thereby resulting in a BSN distance between the next oldest outstanding block transmission and the next RLC block pending transmission falling below the WS limitation. FIG. 1 illustrates transmit window size, WS, and Sequence Number Space, SNS, for an RLC block transmission scheme, also known as an RLC engine. In the following, sequence numbers referring to absolute BSN numbers determined by the RLC engine are denoted BSN [A-D]. RLC engine sequence numbers that are mapped to specific BSN fields in the RLC/MAC header are denoted BSN [1-4].
In GSM/EDGE the maximum WS of the RLC protocol is either 64 or 1024 depending on the feature used. GPRS uses a window size of 64 while EGPRS and EGPRS2 use a window size of 1024. The SNS for the respective features are 128 and 2048 respectively, see 3GPP TS 44.060.
The Block Sequence Number, BSN, for each RLC block transmitted is included in a header of a Radio Block. There are between 1-4 RLC blocks in a Radio Block depending on the Modulation and Coding Scheme, MCS, used. Each RLC block includes an RLC/MAC header+RLC data.
The principle of indicating which Block Sequence Numbers, BSN, that are associated with the RLC blocks carried within a Radio Block is described in 3GPP TS 44.060.
In general terms, if a too small transmitting WS is used in a given RLC block transmission scheme, i.e. an RLC engine, and the feedback from the receiving entity is not fast enough, the RLC engine can stall. Stalling implies that further transmission of new RLC blocks is stopped whenever the transmitter determines that the separation between the oldest transmitted RLC block not yet acknowledged by a receiving entity and the next RLC block pending transmission becomes larger than the window size.
Reasons for stalling can, for example, be that the feedback from the receiving end:                is limited in size (i.e. the BSN space spanned by a given instance of feedback is too small)        not frequent enough        high error probability on the radio link (i.e. the feedback from the receiving entity does not reach the transmitting end).        
The more RLC blocks that are transmitted per transmission time interval, TTI, the higher risk there is for stalling. Window Size 1024 used for the RLC protocol was designed for EGPRS operation where at most 2 RLC blocks per radio block could be transmitted per TTI. Since then, the specification has evolved in features supporting up to 4 RLC blocks per radio block (EGPRS2-B feature) and up to two carriers (Dual carrier downlink feature). Thus, the transmission rate of RLC blocks per TTI has quadrupled.
Currently the multicarrier downlink feature is being specified where the number of carriers will be extended to at least 4 carriers. Also, discussions are ongoing on introducing 2×2 MIMO to enable further improved spectral efficiency and increased end-user throughput. Thus, from the EGPRS single carrier that the RLC window size was designed for there can now be a 2(EGPRS2)×4 (multi carrier)×2 (MIMO)=16 times increase in the maximum transmission rate per TTI.